Dielectric wave guide system



Sept. 9, 1952 M. D. FISKE 2,610,249

DIELECTRIC WAVE GUIDE SYSTEM Original Filed Sept. 15, 1942 2 SHEETS-SHEET l Inventor: Mi lan D. Fl'ske,

His Attorney.

Sept. 9, 1952 D. FISKE DIELECTRIC WAVE GUIDE SYSTEM 2 S!-IEETS--SHEET 2Original Filed Sept. 15, 1942 Fig.7

Inventor:

Milan D. Fiske, flwijwiam His Attorney.

Patented Sept. 9, 1 952 DIELECTRIC WAVE GUIDE SYSTEM Milan D. Fiske,Schenectady, N. Y., assignorto. General Electric Company, a corporation;of

New York Original application September 15, 1942, Serial No. 458,423.Divided and this application December 16, 1944, Serial No. 568,512

The present invention is a division of my copending application, SerialNo. 458,423, filed September 15, 1942, and relates to dielectric waveguides of the hollow-pipe type for transmitting electromagnetic waves ofultra high frequency, and more particularly to structures associatedtherewith'for selectively controlling the propagation of electromagneticwaves through the guides.

It is now quite generally appreciated that electromagnetic waves may bepropagated through the interior of a hollow pipe type guide which may beconstructed of a conductive material and which contains a dielectricmedium such as air or a gas through which the energy of the waves istransmitted. Of course, the frequency of the exciting electromagneticwaves must be greater than the critical minimum frequency established bythe dimensions, principally the transverse dimensions of the guide.

There may be placed within the interior of a dielectric guide a metallicwall having an aperture which is tuned to the frequency of the excitingwaves to effect a concentration of the potential due to the waves withinor across a substantially small dimension of the aperture. As theintensity or the magnitude of the exciting electromagnetic waves isincreased, the magnitude of the potential difference appearing betweenopposing surface of the aperture is also increased, and when themagnitude of this potential difference increases to a predeterminedvalue established by the configuration of the aperture and thedielectric strength of the medium surrounding the aperture, an electricdischarge is established across the aperture thereby materially changingthe propagation characteristics of I the guide.

One way in which a structure of this nature may be employed is inconnection with a system which is subjected to electromagnetic waves ofconsiderable range of intensity and utilizing the characteristics of theaperture to effect transmission or attenuation of the waves within theguide.

It is an objectof my invention to provide a new and improved dielectricwave guide of the hollowpipe type and associated structure whereby thetransmission of electromagnetic waves or the attenuation thereof withinthe guide may be selectively controlled. l

'It is a further object of my invention to provide a new and improvedresonant aperture within a metallic wall enclosed within or associatedwith a wave guide of the dielectric type, and which is enclosed withinan atmosphere, such as a gas, at low pressure. I

9 Claims. (01. 178-44) It is a still further object of my invention toprovide new and improved arrangements for controlling or adjusting thenatural resonance frequency of an aperture mounted within a hollowpipewave guide of the dielectric type.

It is a still further object of my invention to provide a new andimproved assembly for maintaining. a resonant aperture in a dielectricwave guide in an atmosphere of desired pressure, and which isproportioned so that the reflection of the electromagnetic waves isreduced to a minimum throughout that range of intensities within whichit is desired to propagate the waves.

Briefly stated, in the illustrated embodiments of my invention I providea metallic wall having an enclosed resonant aperture, such as a resonantslot, tuned substantially to the frequency of the excitingelectromagnetic waves and which i designed to effect a breakdown of thesurrounding atmosphere in the event the intensity of the electromagneticwaves attains or exceeds a predetermined value. The presence of theelectric discharge across the slot varies the eifective dielectricconstant of the dielectric medium through which the electromagneticwaves are propagated, thereby changing the wave guide from a propagatorto an attenuator of the electromagnetic waves. A structure for housing atuned aperture in a dielectric wave guide is provided which is sealed tomaintain the aperture in an atmosphere of low pressure, and which isalso provided with externally accessible adjusting means to control ortune the aperture.

For a better understanding of my invention, reference may be had to thefollowing description taken in connection with the accompanyingdrawings, and its scope will be pointed out in the appended claims. Fig.l diagrammatically illustrates one way in which my invention may beincorporated in a hollow-pipe dielectric wave guide, and Figs. 2 and 3are views of the sealing structure for the resonant aperture or slot.Figs. 4 and 5 diagrammatically illustrate another modification of myinvention wherein the sealing structure constitutes a dielectric windowof plane construction. Fig. 6 represents a further modification of myinvention wherein a bulbous vitreous dielectric window may be attachedor sealed to the frame including the resonant aperture by means of asealing ring having a temperature coeificient of expansion substantiallyequal to that of the dielectric material employed. Fig, 7 represents anassembly of the elements shown in Fig. 6, and Fig. MS a still furthermodification wherein the enclosing compartment for the resonant aperturecomprises two bulbous type vitreous windows mounted on opposite sides ofthe metallic wall. Fig. 9 diagrammatically illustrates a still furtherfeature of my invention wherein the resonant element is provided withmeans for controlling its effective dimensions, the means beingexternally controllable. Fig. 10 diagrammatically illustrates anothermanner in which externally accessible means may be employed forcontrolling the natural resonance frequency of the slot and wherein theadjustable means is positioned so that it is notsubjec'ted to theelectric discharge whenthe magnitude of the waves exceeds apredetermined value.

Prior to a detailed description of the various embodiments of myinvention, it is believed that it will be helpful to describe'generallythe type of system to which my invention is applicable. It isnow understood that ultra high frequency electromagnetic Waves may bepropagated dielectrically through hollow-pipe type guides where thefrequency of the exciting electromagnetic waves isgreaterthah criticalminimum frequcncy'whi'ch',in turn, is determined principally by thetransverse dimensions of the guide. The guide is preferablyconstructedof a conductive material such as copper or brass. I Many typeser highfrequency electromagnetic waves may be transmitted dielectricallythrough hollow-pipe type guides. "I hese waves have been generallyindicatedheretofore as being of the E andrr typesj subscripts, asindicated, E u and Hmm, have b eeriassigned to these Waves in order toindicate the nature" of the waves, where n represents the order of thewave and m represents the mode of propagation. The order of the waveisdetermined by the manner in which the field intensity variescircumferentially around the axis of the guide, wherea's'the mode isdetermined by the manner of its variation with radial distance from axisof the" gu de, he type Waves h aveb'oth longitudinal and a transversevcomponent of electricv field'but only a transverse ch n or ent ofmagnetic field, whereas the H type waves have both a lpngitudi'nal and atransverse component ofv magnetic field but. only a transwmw ol i Qt. let eld- Although my nvent n abp ica lsjt sy t s for transmitti s a greavar e y wavesl in c in my inv nt en re naft r ref ren w be made i culi'lr t he H 1 e Qiwa e- Referring now to Big. 1 of the accompanyingdrawings, my invention is there illustrated as applied to a systemincluding a hollow-pipe type wave guide of the dielectric type. Thedielectric Wflff guide may be, of rectangular crosssection formed byconductive material, such as copper or brass, havinsa height a and abase dimension b, 'E le'ctn'magnetic waves are established within the gude by suitable exciting, electrodes which may have variousconfigurations, depending upon the nature of the excitation required ordesired. "One form which the input electrodes or exciting means mayassume is that of a concentric line including a, tubular. conductor Iand a conductor}, the; former of which is conductively connected; to,the, bottom of, the wave guide and the latter 6f. Which may beconductively connected to'jthe' top, There is positioned within the waveguid a metal wall. 3,, preferably constructed. of cop ero'r brass, andwhich is provided with'an ap, refd, having an appreciable dimensiontransv'erl's to'the direction of propagation of electromagnetic: wavesthrough the guide. Where the waveguide is'. excited byelectromagneticwaves of the type, for example, the slot affords a principal dimensiontransverse to the electric component of the electromagnetic field andeffects a concentration of the potential due to the wave across theupper and lower edges of the slot. This accentuation or concentration ofthe potential due to the wave is effected by virtue of the fact that theslot is tuned to the frequency of the exciting waves. If desired, theresonant aperture may be of the configuration illustrated wherein anelongated slit 5 is provided with enlarged openings, such as circularopenings 6 and l, at each end thereof, and the complete opening oraperture is tuned to be resonant at the frequency of the exciting waves.

The wall, 3 and the associated dielectric window constitute one end ofthe sealed chamber or section of the dielectric wave guide, the otherlongitudinal end of such section being provided by the end wall beyondthe concentric electrode means I and 2., Of course itis to be understoodthat the concentric line may be sealed in any conventional way so thatthe region between the 'vjall t and the end wall may be maintained at adesired low pressure.

Where'the principal transverse dimension of the aperture isperpendicularto the electric com pcnent of the electromagnetic field,the dimensions of a rectangular aperture may be calculated withreference to the dimensions a and b of the wave-guide. Generallyspeaking, as the effective dimension of the aperture transverse to theelectric component of the field is increased, the frequency to which'theaperture is resonant is decreased. Furthermore, a the effective heightof the aperture approaches zero, the effective length of the apertureapproaches the limit I where A is the wave length of the excitingelectromagnetic waves.

' Where it is desired to seal a portion of a dielectric wave guide or tomaintain the aperture within an atmosphere at the proper pressure inorder to effect a selective response in accordance withthe magnitude of.the electromagnetic waves being transmitted, I provide a sealingstructure for the aperture which performs the scaling function withoutsacrificing the wave" transmitting characteristics, of the guide, Forexample, in Fig. l l provideja bulbous type dielectric window '8; forthe resonant aperture and which is designed to neutralize substantially,any reflection or discontinuity in the, wave propagating characteristicsof the guide incident to the wall 3 and the aperture 4. Stated in otherwords, the bulbous dielectric window. 3 produces an effect which issubstantially equal and opposite to that of the wall 3, and theaperture'so thatthe. amount cfenergy reflectedisminimized. h

The. manner in which he. dielectric windowt cooperates with the wall 3.and the associated aperture to minimize any discontinuity in. impedanceof the dielectric guide may be more fully appreciated by reviewing.certain fundamental concepts. rel'ativeto, the. propagation of an H01 ye, wave in a dielectric. guide. Thetotalimpedance Z0 of a, rectangularwave guide, which may, of course, be characterized as the ratio of theapplied voltage V to the current-I,mayalso be defined as follows;

.V emrc. a 'T 2 it (1) where co is the dielectric constant o f air i-the.

Wave-length of the wavewithin the guidaand c is the velocity of; light.If the total guide. im

M: constant k an dfvvhere his the wave length of the exciting Waves infree space and where the electric component of the fieldisperpendicular. to thedimension bof the guid V I Where .the transversewall includes an aperture which is-resonant to the frequency of theexciting electromagnetic waves and where the change in ,the guidedimensions occasioned by such a structure isa small fraction of the wavelength ofthe waves transmitted through the guide, the

change in thetotal impedance ofiered by the guide is also relativelysmall. I

Therefore, I. provide a window 8 which offers an impedance effectsubstantially equal and opposite to that produced by the resonantaperture, so that a wave propagated through theguide experiencessubstantially no reflection.

The voltage or potentialdifference appearing between the upper and loweredges of the aperture is effected by the resonant characteristics ofthe'aperture, and the magnitude of this voltage differenceincreases asthe magnitude of the exciting Waves increases. Uponreaching a predetermined value, the dielectric material within the aperture breaks down;that is, the voltagediiference is suflicient to cause ionization of themedium which may be air, and an electric discharge takes place acrossthe aperture. Upon occurrence of such an electric discharge, the wavetransmitting characteristics of the guide are radically changed, causingrapid attenuation of the electromagnetic waves within the vicinity ofthe wall 3. Without attempting to enter into any detailed discussion ofthe reason why the waves are not transmitted beyond the wall 3, sufficeit to. say that the presence of the charged particles within thevicinityof the resonant aperture, due tothe electric discharge,effectively changes the dielectric constant of the medium through whichthe waves are being propagated, as, for example,. air, in the presentdiscussion, thereby substan tially increasing the critical minimumfrequency fonthe particular guide employed so thatthe waves are-notpropagated an appreciable distance beyondthe wall 3.

Fig-2 represents a more detailed cross sec.- tional view of that portionof the Wave guide including the wall 3 ofthe dielectric Window 8. The,dielectric window 8 may be constructed of a suitable vitreous material,such as a boronsilicate glass, and sealed .to the metal wall 3 by meansof a cylindrical flangev 9. The flange 9.

may be constructed of an alloy, such as an ironnickel-cob'alt alloy, tofacilitate sealing of the glass to the metal wall.

In Fig.3, an end view of the bulbous dielectric window and the resonantslot is also illustrated. Figs. 4 and5 represent two views of adifferent form or the dielectric window for sealing a section of adielectric wave guide... The metallic wallincluding the resonantaperture or slotmay .be formedintegral with. the walls of the dielectricwaveguide, as shown in Fig. 4, where the walls oonstituteright-angleextensions. of the top l0 and the bottom of a sectionof a rectangularwave guide. In this modification of my invention, the resonant aperturemay be in the nature of. a tuned or resonant rectangular slot 12 whichisalsotuned to the frequency of the exciting means; .which establishes theelectromagnetic waves in the guide andin which the height h and thelength L thereof bear a. definite relationship with respect to theexcitation frequency and the transverse dimensions of the guide,particularly the base dimension b. It is to be understood that variousdimensions of the resonant slot may be. employed; that is, thedimensions of the slot may be varied and still maintain its resonancewith respect to the exciting frequency.

I provide a sealing structure for a dielectric wave guide which not onlyaffords mechanical strength where it is desired to seal a section of adielectric wave guide, but is also designed and arranged so that theassembly including the slot,

tom 1 l of one section of a wave guide and a top I6 and bottom I! ofanother section of a dielectric wave guide. It is to be understood thatsimilar strapsMaand l5a shown. in Fig. 5 are also placed. along thesides of the guide to afford similar bridging means. In this manner, acontinuous conductive path is provided between the two sections of theguide. If it is not desired to form the wall or diaphragm #8, whichincludes the resonant slot l2, integral with the top and bottom membersl0 and II, the diaphragm may be soldered or welded to these members.

The window 13 is designed and positioned relative to the wall 18 so thatthe reflection from the slot l2 cancels the reflection from thedielectric,

or vice versa, depending upon the direction of wave propagation throughthe guide. The window I3 may be cemented to the diaphragm by using avacuum cement I9 so that one of the sections orregions of the wave guidemay be maintained at a different pressure from an adjoining region.

I have found that the length L of the slot :2-

required to make the window [3 reflectionless depends very criticallyupon the proximity or spacing of the dielectric material used in thewindow. To establish this distance and to allow space for the cement [9,a thin metal gasket or spacer 2D is inserted. around the slot 12. Thisgasket may be formed as an integral part of the wall or diaphragm I8 andpositioned as close to the slot-as desired. Residual reflection from thewindow [3 after construction may be canceled or neutralized by cementingto the outside surface of the dielectric window l3 a diaphragm l3 ofproperly chosen characteristics.

I have found that a window constructed in accordance with the principlesof my invention de-.

acre; 249

0.3 %1offthe energy incident upon it...

Figs. 6iand i7 diagrammatically .illustratexa still further modificationof ithe-WindOWl COIlStlJllCtlOll ion a a dielectric waveguide wherein;ammetallicwall member 2| positioned ina..dielectric.wave. guideand"preferablyconstructed of a;metal,..suc-h.

as brass or copper, is provideid with a resonant apertureiZZL' of thetypezhavingan elongated slit'i" with circular- Openings ateitherend:Inzorder to establish awseal :of' suitable mechanical; rigidity.- and-one which is substantially reflectionlessyby' maintainingthedielectricelement. of the -.window. preciselypositioned relative to thewall and. the

slot, I pro'videan: arrangement wherein the di electric. window-sissealed to the :walli'by means: of.

a supporting frame or ring-23," preferablyrco'nstructed ofa metal suchas an iron-nickel=cobalt alloy, thereby. facilitating the. sealin of a..Vitres ousor: glassdielectric. window 24. whichxis of. bulbousorhemisphericalconfiguration.- The mes tallic ring 2 is preferablyconstructed; of a .thin sheet and may be formed by a suitable stampingoperation so that it is. provided withitwo sections 24a and 24b ofunequal'diameter joined by 215L113! stantially transverse section25:1so. that a certain degree of resilience is provided to compensatefor strains incident to pressure. and temperature variations. In thismanner, variations in pressure do not change the position of theglass'window, and consequently do not appreciably affect the netreflection from the window. Furthermore, the glass Window 25 isconstructed of a suitable glass, such as boron-silicate, having atemperature coefficient of expansion substantially equal to that of themetal constituting the ring 23. Wall 21' is provided with an annulargroove 2'! inwhich the section 24a of ring 23 is seated. Ring: 23'may bewelded or soldered to wall 2 l.

In the production of this type of window; the ring 23 may be sealed tothe wall 2! and the glass wihdowZB may be sealed toscction 24b of thering.

by placing thewindow 26' upon the section 24b and subjecting the rin 23to a .heatingiagency such as a high frequency field, causing the-glassto seal around. the outer edge of section 24b,zthe

-vide externallyaccessihle means for varying the natural resonancefrequency'of the slot, and which may comprise a screw QEIWh'lCh' extendsinto the body of the Wall 2| and intotheaperture; and as illustrated,may extend into one of the circular openings at the ends-of theelongated. slit 22 thereof. It will be appreciated that as the positionof the. screw is changed, the .efiective dimensions of the aperture arealso changed, thereby-adjusting or controlling the frequency at whichthe aperture is resonant.

If desired; the screw ZBinay he provided Withan extension 28" ofsubstantially paddle shape which may extend into one of thecircularopenings. of the resonant slot,a.and the position of which determinesor, controls the natural resonance frequency of the slot.

Where it is desired to -maintain the resonant aperture in a suitableatmosphereand at.the

proper pressure, thus maintaining ahigh degree s of sensitivityofatheaaperture tolthe intensity-of the; electromagnetic waves; being:transmitted; through the guide; the. aperture; may: be come pletelyenclosed inxsucn an" atmosphere: For? example; two: arrangementszsuch'as. shown in:

Fig.1fi may be:employed one'on each sideofi the: wall 2 I, in a manner.illustrated in: Fig; 8'.'wher.e;=* in a wall 29, provided witharesonantraperturez- 30, is sealed by bulbous vitreous windows 3| and32,,which are sealed to the wall 29 by rings 33 and 34 of a constructionand material similar to ring 23 discussed :inconnection with Fig-,6.-

The atmosphere employed within-the enclosure may be any suitable fluidor gas which will sup- 3 port an electric discharge when the voltagedifference between the edges of theslot or aperture attains apredetermined value; and which- -per=-'- mits a substantially rapid Iextinction of the arc when the voltage difference decreasesbelowf thecritical value. For example, I may employwith intheenclosurean-atmosphere-of hydrogen-and water vapor; an atmosphere of nitrogen, oran atmosphere of argon.- I

It'will be understood that in-- the application 4 of the embodiment ofmy invention shown in Fig. 8; to a dielectric waveguide, wall member 29is placed in conductive contactwith the metallic or conductive walls ofthe guide; For-example," it -may replace the wall 3 -andassociatedassembly shownin Fig. 1. 5 As-statedabove, the dimensions of a resonant'slot in a'system of this nature-are fairly critical} that is, variationsin thedimensions--cliange-the frequency at which theaperture or slot*is' resona-nt. In someapplications' and in the operation of-dielctricwave-guides, v it-may-be de-- sirable to tune-or adjust the frequencyat-which the aperture is resonant while maintaining-the aperture ina-region of low pressure or in a-re'g ion ofa desired atmosphere.Consequently; I' pro vide externally accessible -means for-controllingthe effective dimensions-Ora tunable aperture mounted in;- ametallicwall suitable for "use ';in-- connection with-a dielectric wave guide.One; form ofmy invention is illustrated-in Fig-g 9 where in 'ametallicwall 36 is employed and may beconstructed inupper and lower sections 31and-38 1 to facilitate the construction of theapertureand to permit theplacement therein ofthe adjusting means to be described presently;-

Theaperture 33-maybe of the farm illus trated inFig. Tcompris'ing arestricted "-elong'ated section-andhaving circular openings at eac'liend thereof; Furthermore, the aperture-'33 may- Q be maintained at a-low-'pressure,- ormaybe m-aintained inasuitable atmosphere at theproper pressure by means Ora anof bulbous vitreouswindows 40 and whichare sealed to and sup-- ported by the W alI SB by ;means--of metallicrings 42- and 43' constructed in the mariner explained above inconnection with the apparatus shown in Figi6'." The-rings 42 and-43maybe s'eate'd in annular gro'oves in th'e "end fa'cespf wall 36.

As a means for tuning the resonant aperture 39'; I provide adjustablevmember; suchflas :a.v

deformable diaphragm' l i' which -may constitutes one boundary of :the.resonant aperture and may; be welded or.solderedztoitlietupper section3170f; Wall136; i

The uppersecti'orr 3710f .Wall 36;.isprovi'ded-Zvvitlii: a recess45,which may be .of.'rectan'gulari-z-cross'zv section,.and.iscalso providedwithia communicating circular copeningldfi whiclr extends. to tan outerperipheryiiof'ithe:upper'; sectionl'.3"|; l This arrangement "permitsexternal-adjustment of .the

position of the diaphragmsothatit is notneces-i sary'to disturb theseal'ior the resonant aperture in making an adjustment; I provideametallic block" 41 whichis connectedtothe diaphragm 44 andwhich=ispositionedIby means of adiffer ential screw arrangement48 whichextends from the block 41 to the recess 49". The block- 41'is preferablyconstructed "of a 'material having a relatively high thermalconductivity and thermal capacity to facilitate the dissipationof theheat incident to the establishment of electric'discharge across theboundary ofithe resonant aperture when the intensityof theelectromagnetic waves exceeds the critical'value. 1 1

For example, the block-41 may-be constructed of copper. Upon adjustmentofthe difierential screw arrangement 48, the block 41 is raised orlowered, thereby deforming the diaphragm 44 and effectively controllingthe dimensions of theresonant aperture; Of course, thediaphragm 44 issealed toKthe upper: section 31: of wall 36 so that theadjustmentmay bemade without, disturbing the condition of the atmosphere with inwhichthe resonant aperture is enclosed.

An alternative embodimentof my invention is. illustrated in Fig. 10wherein the adjustable means'for controlling the natural resonancefrequency of an aperture or slot 5 is positioned in the slot so as notto be subjected to the electric discharge which is produced when themagnitude of the electromagnetic waves transmitted through a waveuideattains or exceeds a predetermined value. The resonant aperture orslot 50 comprises an elongated portion or slit terminated at each end-in enlargedopenings52 and {53; One of the openings, such as opening 53,may be provided with an extension 54 in which is placed an adjustable ordeformable diaphragm 55 which is sealed to the body of the wall so thatthe region including the aperture may be maintained at low pressure orwithin a desired atmosphere. A metallic member or block 56 of highthermal conductivity is attached to diaphragm 55 and may be actuated orpositioned by a differential screw arrangement 51 similar to that shownin Fig. 9. In like manner, the adjusting means is made externallyaccessible by extending to a recess 58 in the body of the metallic wall.

While I have shown and described my invention as applied to a particularsystem embodying various devices diagrammatically shown, it will beobvious to those skilled in the art that changes and modifications maybe made without departing from my invention, and I therefore aim in theappended claims to cover all such changes and modifications as fallwithin the true spirit and scope of my invention. a

What I claim as new and desire to secure by Letters Patent of the UnitedStates is: i

1. In combination, a dielectric wave guide of the hollow-pipe type,exciting means for establishing electro-magnetic waves within saidguide, a metallic wall positioned in said guide and lying in a planesubstantially transverse to the direction of wave propagation throughsaid guide and provided with an elongated slit having a pair ofsubstantially circular openings at each end thereof, said slit andassociated openings constituting an aperture tunable to the frequency ofsaid exciting means, and a deformable diaphragm forming a boundary ofsaid slit for control1ing the effective natural frequency of saidaperture.

2. In combination, a dielectric wave guide of the hollow-pipe type,exciting means for establishing electro-magnetic waves within saidguide,

a metallic wall positionedin said guide and-lying in a planesubstantially transverse'to the direction of wave propagation throughsaid guide and provided with an aperture comprising a slot hav-'- ingits principal dimension transverse to the elec tric component of thefield incident to'the propagation of waves through said guide andhavingapair of circular openings at each end'thereof, means for providing anevacuated enclosure for that portion of said wall including saidaperture; a deformable diaphragm supported by, said Wall andconstituting a boundary of said slot for controlling the effectivedimensions ofsaid-ap'erture'; and externally accessible means: foradjusting said diaphragm. 1 Ti H i l-f lz 3. In 'combinationa dielectricwave guide of the hollow-pipe: type, exciting means for estab lishingelectro-magnetic waves within said guide; a metallic wall positioned insaid guide and lying in a plane substantially transverse to the direction of wave propagation through said guide-and provided with anaperture comprising a slothav' ing its principal dimension transverse tothe ele'c trio component of the. field incident to the propa gation ofWaves through said guide and having a pairof circular openings at eachend thereof, meansfor providing an evacuated enclosure for that portionof, said Well including said a'perture, said wall being provided with arecessiadjoining said slot; a diaphragm constituting -a boundary of saidslot and attached to said wall, a metal block of sub'stantial' thermalcapacity and conductivity in physical contact with i said dia' phragm,and externally accessiblemeans for adjusting the position of said 5block and said dia-' phragm l ,l l

4. In combination, a dielectric wave guide of the hollow-pipe type,exciting means for estab lishing electro-magnetic waves within saidguide, a metallic wall positioned in said guide and lying in a planesubstantially transverse to the direction of wave propagation throughsaid guide and provided with an aperture comprising a slot having itsprincipal dimension transverse to the electric component of the fieldincident to the propagation of waves through said guide and having apair of circular openings at each end thereof, means for providing anevacuated enclosure for that portion of said wall including saidaperture, 7 said wall being provided with a recess adjoining said slot,a diaphragm sealed across said recess and constituting one boundary ofsaid slot, and externally accessible means for adjusting the position ofsaid diaphragm.

5. In combination, a dielectric wave guide of the hollow-pipe type,means for establishing electromagnetic waves within said guide, ametallic wall associated with said guide and lying in a planesubstantially transverse to the direction of wave propagationtherethrough and including a resonant aperture comprising an elongatedslit havin at one end thereof an enlarged opening, a diaphragm forming aboundary of said opening, and means extending through the body of saidwall for positioning said diaphragm.

6; In combination, a dielectric wave guide of the hollow-pipe type,exciting means for establishing electromagnetic waves within said guide,a metallic wall adaptable for use in connection with said guide andhaving a plane substantially transverse to the direction ofwavepropagation therethrough and provided with an aperture tunedsubstantially to the frequency of said exciting means and comprising anelongated slit terminated in an opening in one end thereof,

and :means; for: controlling "thenatural frequency 01 said. aperture.icluding a member extending; into said: opening andactuatingmeans forsaid member-'lying'within thebody of said metallic wall.- I

*7. :In-asystem. for the transfer of high ire-- quency; electromagneticwaves including a pipe waveguide a protective electricalbreakdown-device which-.includesa metallic conducting barrier locatedtransversely of said wave guide and havinganaperture of elongatedcontour with its long. dimension oriented perpendicularly to theelectric. vector of: oscillation transmitted in said waveguide,- thewidth of said aperture being sufficiently small at least for a part ofits length to permit theoccurrence of. an electrical breakdown dischargeacross said aperture when oscillations in. said guide exceed apredetermined amplitude, said aperture. being dimensioned and shaped. toresonate. at. a frequency in the=neighw borhood of .that of oscillationsdesired to be transmitted through said Wave guide system, and arotatable vane within at least alportionof said aperture for adjustingthe. resonant frequency thereof.

8. Ina-wave guide system, a protective device which includes aconducting barrier positioned transversely of the direction of,propagation of electromagnetic energy in said Wave guide and havinganelongated aperture located in said barrier, said aperture bein sodimensioned in a direction parallel to the direction ofthe electric heldof oscillations in said guide that a break.- down will. occur acrosssaid aperture in said direction when the amplitude of said oscillationsexceeds a predetermined value, said aperture be ing further dimensionedwhereby said. aperture: is resonant at: the frequency; of saidoscil1atlons,

quency electromagnetic waves including a. pipewave guide, a protectiveelectrical. break-down device whichincludes a metallic conducting: bar.-

rierv located. transversely of said wave guide and having an aperture ofelongated contour with. its. long dimension oriented. perpendicularly tothe electric vector of oscillations transmitted in said wave guide; thewidth of said aperture being sufficiently small. atleastfor. a partofitslengthf. to. permit the occurrence of an electrical break downdischarge across saidaperturewhen oscillations in said guide exceedapredetermined. amplitude, said apertureqbeing dimensioned and.

shaped toresonatesat a frequency in; the neigh 'lborhood of that. of:oscillations desired.- tohe, transmitted throughnsaid wave guide,system, and.

means for tuning said.- apertured barrier for: ad.- justing the.-frequency: at which. it. resonates.

MILAN. D. FISKE.

REFERENCES CITED The following references are of record in the file ofthis patent? UNITED STATES PATENTS Number Name Date- 2,396,0&4 Fox .Mar.5, 1946 $102,184. Samuel .J.une- 18, 1946

